The present paper interpreted the traditional stress-strength interference model as an equation expressing the statistical average of the probability of strength preponderating over stress. Thus, the same equation, which was traditionally applicable only in the situation of same-measure parameters, can be applied to more general situation of different-measure parameters. In other words, the traditional model was extended to the situation of any two variables, as long as one variable can be expressed as a function of the other. With a specific load amplitude distribution, the method is to calculate the statistical average of the probability that fatigue life random variable under the individual constant amplitude cyclic load is greater than an assigned value. The extended interference analysis method can be applied directly to calculate fatigue reliability under constant amplitude cyclic load with uncertainty in the stress range.